Genetic analysis of inbreds, inbred crosses and hybrids of cocoa (Theobroma cacao L.)
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Date
2019
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Department of Plantation Crops and Spices, College of Horticulture, Vellanikkara
Abstract
The genetic analysis of inbreds, inbred crosses and hybrid was carried out at College of Horticulture and Cocoa Research Centre (CRC), Vellanikkara, Thrissur. A total of 113 inbreds was evaluated for qualitative and quantitative characters. Cocoa is predominantly out breeding with highly complex genetic structure. The cross pollinating nature coupled with existence of self/cross incompatibility, poses much difficulty to the cocoa breeders. The hybrid progeny from the same cross exhibit high level of variability due to heterogeneous nature of the parents. This can be overcome to certain extent by using fully homozygous inbreds of diverse genotypes. The CRC succeeded in producing first ever fifth generation inbred and it was proved self incompatible. In this context, the present study was formulated to quantify the magnitude of inbreeding depression in yield and yield attributes in various self-generations and to establish a physiological relationship between the vigour of inbreds, inbred crosses and hybrids in the early stages of plant growth. The different pollination techniques were tested to overcome the self incompatibility and the extraction of proteins from self incompatible plants was attempted. Morphological characterization of 113 inbreds were carried out by recording qualitative characters such as pod shape, ridge colour, pod apex, pod basal constriction, pod rugosity and bean colour. High variability was observed for all qualitative characters except ridge colour. The pod and bean characterization for 21 characters expressed wide variation among the inbreds and within the inbreds of same genotype. Characterization of inbreds based on the biochemical parameters such as fat and phenol expressed wide variability. The inbreeding depression was estimated for 21 characters in 12 genotypes over generations. In S1 generation of M 18.7, the husk furrow thickness and number of flat beans expressed negative inbreeding depression, whereas the average yield per tree per year has shown a very meager inbreeding depression over the preceding generation. In S2 generation of M 18.7, pod value, conversion index and dry matter recovery have shown an inbreeding depression between 5 to 10 percent. Negative inbreeding depression was observed for husk ridge thickness, number of flat bean per pod, pod index, efficiency index and fat content. In S3 generation of M 18.7, maximum inbreeding depression was observed for wet bean weight per pod followed by pod length. In S4 generation of M 18.7, the highest inbreeding depression of 43.58 per cent was observed for number of pods/tree/year. In S1 generation of G II 7.4, positive inbreeding depression was observed for 10 characters out of 21. In S2 generation of G II 7.4, positive inbreeding depression was observed for 15 characters. Only seven characters expressed positive inbreeding depression in S3 generation of G II 7.4 genotype. In S5 generation inbred, the maximum inbreeding depression (63.95%) was observed for husk furrow thickness, followed by husk ridge thickness (49.02%), In S1 and S2 generation of H1 1.2, positive inbreeding was observed in 16 characters. In general, inbreeding depression was less for economic characters confirming that most of the characters are controlled by additive gene action and lethal gene canceled in heterozygous condition is less. In the comparative evaluation of inbreds, inbred crosses and hybrids, the inbreds were found superior over inbred crosses and hybrids for morphological characters such as plant height, collar girth, plant spread and leaf area. The inbreds also had maximum chlorophyll content, leaf nutrient status and relative water content. The superiority of inbreds was mainly attributed to the growing environmental condition with more of openness in the inbred plantation, which was confirmed with spherical densiometer, an instrument for measuring plantation overstory density. A total of nine techniques to overcome self incompatibility barriers such as bud pollination, surgical technique, intra-ovarian technique, salt spray (1% and 3%), high humidity, high temperature, Naphthalene Acetic Acid (100 and 200 ppm), gamma irradiation and flower organ extract were attempted from September 2016 to March 2017 and September 2017 to March 2018. In all the methods, the fruit set was not obtained except in surgical technique. The majority of self incompatible S3 and S5 inbreds set in to pods through surgical techniques. In the fluorescent microscopic study, the clear disintegration of pollen tube was observed in self incompatible flower in the style. Flowers from fifth generation cocoa inbreds were used for extraction of protein. All the available protein extraction methods were tried and the protein was quantified. In all the methods, the protein content was below 5μg per ml in the cocoa flower. The protein from cocoa leaf was extracted and quantified. In lysis buffer extraction with some modification, a protein content of 7 μg per ml was obtained. Faint bands were observed in SDS PAGE. The flower and leaf protein were subjected for LC Q ToF. The protein content obtained from the flower was insufficient to run LC Q ToF. A total of 87 proteins were found in cocoa leaf sample, of which, 85 proteins were similar to cocoa and two proteins were similar to forest coconut. From the above, it can be concluded that the genotypes in S5/S6 can be crossed to get highly heterotic hybrids. For extraction of protein from cocoa flowers, fine tuning of the available methods under ideal laboratory conditions must be employed. Molecular basis of self-incompatibility has to be studied in detail.
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174734